GB2252315A

GB2252315A - Fiber-reinforced ceramics

Info

Publication number: GB2252315A
Application number: GB9126343A
Authority: GB
Inventors: Hermann Koeberle; Kilian Peetz; August Muhlratzer; Hartmur Greulich
Original assignee: MAN Technologie AG
Current assignee: MT Aerospace AG
Priority date: 1991-01-31
Filing date: 1991-12-11
Publication date: 1992-08-05
Anticipated expiration: 2011-12-11
Also published as: FR2672239B1; GB2252315B; FR2672239A1; GB9126343D0; DE4102909A1

Abstract

A composite fiber structure (10) has fiber inserts with different materials or structures (13 through 17) and a ceramic matrix. The fiber structure comprising the fibers inserts is impregnated with the matrix material by liquid phase or gas phase infiltration, optionally after a prior treatment of the composite with a liquid polymer material. The fiber inserts may be of carbon, silicon carbide or alumina fibers, e.g. in fabric form, and the matrix may be of silicon carbide, alumina or mullite. The manufacture of components for a jet engine air intake, pumps and an injection molding machine, as well as brake discs, is described. <IMAGE>

Description

2,-;, 1 - 1 - i 1 Workpieces of Fiber reinforced ceramic Material The

invention relates to workpieces manufactured of fiber reinforced ceramic material comprising at least two ceramic fiber inserts which are juxtaposed and surrounded by matrix and to a method for producing such workpieces.

Workpieces of this type are more particularly utilized in high temperature applications. So far such fiber reinforced ceramic components made up of fiber inserts or cores with a given weave or structure and composed of a given fiber material were stacked on top of each other and impregnated with SiC matrix material. In this case the structures utilized were for instance plain, satin or twill weave fabrics or simply unidirectional fibers were used for stacking in the case of fiber material of carbon or SiC fibers. The selection of the fiber structure also determined the mechanical properties of the material for a given combination of fibers and matrix.

In fact the use of a certain type of fabric or wound structure in accordance with the prior art sets the properties of the material (such as tensile strength, compressive strength, rigidity and anisotropy) within narrow limits. It has consequently been seen that such components are not suitable for many complex applications, as for instance for the production of light-weight load bearing components.

Accordingly one object of the present invention is to provide workpieces of the type initially mentioned which embody a wider range of properties.

In order to attain this and/or other objects in accordance with the present invention the fiber inserts have different structures and/or consist of different materials.

1.1 The composite fiber ceramic structures in accordance with the invention are preferably characterized in that they are built up of different fiber structures and as a whole are impregnated with a ceramic matrix material.

Owing to the particular structure and the combination of different fiber inserts the properties are improved over those of composite materials composed of similar stacked fiber inserts impregnated with SiC matrix material with a uniform, constant type of structure and there is a wider range of application. A further advantage to be seen in the use of different fiber structures within a ceramic component is the substantially enhanced freedom for the designer of a component of fiber reinforced ceramic material. Therefore it is possible for the properties of the component of fiber reinforced ceramic material to be adapted in an optimum manner to the requirements set. For instance, using a combination of dense fabric surface with a voluminous but wide mesh fiber structure it is possible to produce a balanced relationship between strength and rigidity with minimum weight. The invention consequently serves to improve the properties of fiber reinforced ceramics and more particularly for applications in light-weight systems.

The invention also relates to a method for the production of workpleces, wherein a fiber insert of ceramic material is produced and laid together in the form of a composite fiber structure, is impregnated with a ceramic matrix material and is fired, said fiber insert having different structures and/or different materials therein.

Dependent on the intended use the fiber structures may represent a combination of plain woven fabric, twill woven fabric, or a polar woven fabric or of non-woven structures or other inserts with unorientated fibers (felts) or orientated fiber (for example wound structures or unidirectional laminates). The individual structure elements (the fiber inserts or cores) may have different degress of compactness and consist of different fiber material (carbon, silicon carbide, silicon-carbon nitride, aluminum oxide and mullite). The selection and the arrangement in the fiber structures will be dependent on the requirements for the intended use or function (impermeability to gases or strength).

It is possible for the composite fiber structures to be laid sandwich-wise in one dimension but they may also be grained or structured laterally in both other dimensions.

For the formation of the ceramic matrix in accordance with the invention methods of impregnation using a liquid phase may be utilized.

9 3 Gas phase infiltration and/or impregnation with liquid materials, which may be in a liquid form with or without solvent, and which function as a precursor of the ceramic matrix, constitute preferred methods.

In accordance with a further possible advantageous development of the invention in a preliminary stage the composite fiber structure is caused to absorb a polymer material either in a pure liquid or dissolved form in order to fix the fibers and thereafter such polymer material is subjected to heat treatment to cure it and then pyrolized. The quantity of the applied material is so selected that the fiber strands are spanned without however leaving the interstitial pores in the bundles of fibers and between the strands completely filled. Therefore a strong connection is formed between the fiber structure elements without the following impregnation with the matrix material being substantially impeded. In order to monitor this operation it has proved satisfactory to measure the drop in pressure of the gas flowing through the structure.

Further advantageous developments and convenient forms of the invention will be gathered from the following detailed account of embodiments thereof in conjunction with the accompanying drawings.

Figure 1 shows a simple component or workpiece in the form of a parallelepiped. Figure 2 shows a cross section (measuring 9.3 by 3.6 mm) with a 15-fold magnification under an optical microscope. Figure 3 is a plan view of a first sample of a material with a wide-mesh multi-layer fabric and a densely stacked laminate of two dimensional fabric elements as seen under a scanning electron microscope. Figure 4 is a 50-fold magnified cross section (measuring 1.6 by 2.2 mm) of a sample produced as in example 2 under an optical microscope.

The invention will firstly be described in detail with reference to figure 1 showing a workpiece or component 10 in the form of a parallelepiped. As shown at two cross sectional surfaces 11 and 12, the workpiece 10 has different structures. These structures firstly consist of different fiber plies 13 through 17 stacked in the z-direction with the two outer plies 13 and 17 and the center ply 15 extending over the full surface or area, while in the y direction the intermediate ply 15 has three structures 14a through c and the intermediate ply 16 has two structures 16 and 16b in the x direction.

The various structures differ with respect to the type of weave of 4 the fibers. Thus for instance it is possible for the continuous plies 13, 15 and 17 to be fabrics of the same or of different type (plain woven, twill etc) while the fiber insert 14a is a felt and the inserts 14b and 16b consist of unidirectional fiber plies with different alignments (in the x and, respectively, the y direction), the two fiber inserts 16a and 14c being fiber wool with a different density.

Further parameters which may be varied include the type of fiber material, the type of matrix material and the density or porosity of an insert. These characteristics may be varied as desired. The selection both of the fiber insert structure, its distribution and furthermore the type of materials will be dependent on the specific application.

One respective ply 13 through 17 may consist of a fiber insert or of a stack of fiber inserts. In the case of production of geometrically irregular workpleces the suitably cut fiber inserts are preferably placed in a mold.

After the juxtaposition of the fiber inserts the composite fiber structure formed as a whole is impregnated with the matrix material and then fired.

It is preferred to fix the fibers within the composite structure prior to impregnation by means of a material such as more particularly a polymer material which is caused to enter the composite structure as a pure liquid or as a solution and is then cured by following heat treatment and the finally pyrolized.

The type of polymer for such fixing of the fiber body will be dependent on the conditions, under which the resulting component is to be used. For applications w ith a operating temperature under approximately 400 C organic resins such as epoxy, phenolic or imide resins or thermoplastic resins are suitable. By pyrolysis they are transformed into amorphous carbon, which is responsible for the strengthening of the structure. For operational temperatures above around 400 C in air a silicon containing polymer such as polycarbosi lane, a polysilazane or a polysiloxane is utilized in the same manner, since although the product resulting from the heat treatment is amorphous, it is resistant to oxidation and essentially consists of SIC. In order to establish an optimum level of matrix for a balanced ratio between strengthening effect and residual porosity, it is possible for the polymer to be dissolved in an aprotic solvent such as hexane.

After the preliminary impregnation and the heat treatment to fix the fiber structure as a composite body filling with the matrix material j takes place by chemical gas phase infiltration (CVI). In the present method it is more particularly preferred to use aluminum oxide (Al.03) derived from aluminum chloride (AICY and carbon dioxide (CO.) in a current of hydrogen, silicon carbide (SIC) derived from trichoromethyls! lane (CH.S'C13) in a hydrogen current and silicon nitride (SI.,N4) derived from silicon fluoride (SiF4) and ammonia (NH.) to produce a matrix in the fiber structure. With more fiber structures as are preferred for instance for producing dense surfaces, the best results are produced with the gradient process (as described in, for instance, W.V.Kotienski "A Review of CVD Carbon Infiltration of porous Substrates", 16th Nat. SAMPE Conf., 1971, p.

257 ff), which is on the basis of controlled flow through the fiber body.

As an alternative to CVI it is possible to produce the matrix by impregnation of the preliminary blank with a liquid phase followed by heat treatment including drying and pyrolysis.

The starting materials utilized for such liquid phase impregnation differ in their chemical nature dependent on which matrix material is used.

For the production of a matrix of SIC the impregnation is performed with a polysilazane. An A1,03 matrix is produced in the so-called sol-gel process from aluminum tri-tertiary butylate by controlled hydrolysis and following drying and sintering.

For the production of oxide matrices, A1.03 or mullite, in lieu of the sol-gel process it is possible to use super-fine oxide powder with a particle size of approximately 0.1 micron to impregnate fiber blanks. In this case however a substantial enrichment in the surface zones with solids was detected. Furthermore hardly any matrix material entered the interstitial space between the individual fibers of the fiber bundles.

However it is necessary for the space between the fibers to be spanned by the matrix material in order to transfer loads in the composite material and consequently to increase its mechanical strength.

Figure 2 shows a cross section of a sample 20, which is to be seen in figure 3 in surface view. The sample consists of a wide-mesh, multi- ply fabric 21 and a densely stacked laminate 22 of bidemensional fabric, each such fabric ply consisting of SIC fibers in combination which have a CVI SIC matrix. While the denser ply 22 in this composite construction serves to provide strength, the buttressed, open-work fabric 21 additionally ensures rigidity and a low weight.

Figure 4 illustrates a cross section with a magnification of 50-fold taken through the sample 30 of example 2, such sample consisting of a stack of pieces of woven fabric with a plain weave 31 and with felt 1 6 inserts 32 of SiC fibers 33. 33 denotes the residual porosity.

These methods were found to lead to advantages when used for the production of test components of fiber reinforced ceramic. In this case the principle in accordance with the invention of the combination of different cut patterns of the fibers in workpieces and the adjustment of their properties after incorporation in the matrix were demonstrated. In what follows such working examples of the invention will be described and the optimum adaptation of the properties of the components to individual requirements by the structure of the invention will be explained.

Example 1

A structural component in the jet engine air intake of a high speed airplane was produced which has a box-like configuration consisting of SiC ceramic material reinforced with carbon fibers in the following manner.

A so-called spaced fabric consisting of carbon fibers cut to the final form and constituting a hollow parallelipiped box with two internal ribs continuously extending parallel to the longitudinal edge and having a multi-ply fabric structure, was drawn taut on a support structure. The fabric inserts of carbon fibers were placed thereon, the plies alternately having plain and satin weaves. The intermediate placement of twill fabric with good draping properties served to minimize the size of interiamellar cavities and to ensure the evenest possible distribution of the residual porosity in the composite material.

The fabric structure was impregnated with a silicon containing polymer, namely a mixture of polymerized silazanes produced from methyld ich lorohyd rogensi lane (CH,CHSiCl.) and viny ltrichlorosi lane (CH2CHSiCH.) with ammonia (NH.) as a cross linking agent. By curing to produce such polymers and then pyrolysis to yield SiC a box-like fiber body was obtained in which the fiber plies were so strongly connected together that it was self-supporting and able to be handled and could be placed in a CVI apparatus. CVI was performed using the gradient method with a gas current of methyltrichloros! lane in H2 woth a maximum temperature of 1150 C.

This structural principle coupled with the manner of production described leads to a component with an optimum ratio of strength and rigidity to weight. The component rigidity necessary for its aerodynamic loading was obtained by the infiltrated out plies with approximately 45 % fiber by vol and a torsional rigidity due to the internal ribs of open pored C/SiC composite material.with a multi-ply fabric having an average fiber content of 20 to 25 % by volume.

0 7 Example 2

SiC fiber fabric with a plain weave and cut into the configuration of a disk was stacked with intermediately placed, thin felt plies of short SiC fiber and dense short fiber mats as covering plies on either side and mounted between perforated graphite plates applying a pressing pressure in a CVI apparatus. Using the gradient method the disk-like fiber body was infiltrated with SiC matrix. In the short fiber intermediate and covering plies the flow conditions obtaining therein during the gas phase infiltration led to a substantially greater growth of the matrix than in the fiber plies so that here it was possible to set a substantially higher matrix to fiber ratio than in the fabric plies (see figure 4). Owing to the pressing of the short fibers 32 substantially into the pores spaces 33 which are not filled by the fiber strands of the adjacent fabric plies, it was possible to attain a generally high compaction during infiltration.

After suitable machining the composite component was tested as a brake disk. Its suitability for this application is the result of the selected fiber structure and is determined by the high compression and shear strength which is then produced in conjunction with and with the resistance to thermal shock of SiC/SiC as a material and its frictional behavior.

Example 3

A fiber member in the form of an elongated cylinder of A120, fiber was produced by cross winding with a diagonal orientation in relation to the axis of the cylinder. Using a multi-ply configuration of the fabric body a wall thickness of 2.5 mm was produced. The stabilization of the initial or preliminary fiber body was ensured as in example 1 by impregnation with a polysilzane mixture and pyrolysis to give nitrogen containing SiC. CVI to produce the SiC matrix was performed in a manner similar to example 1.

On testing the composite ceramic tube produced in this manner in a canned motor pump it was found that the sealing properties of the material with respect to preventing the escape of liquid were insufficient. In a further embodiment of the invention similar to example 2 thin short fiber intermediate plies and a short fiber covering ply were incorporated in a component which in other respects was produced in the same manner as in example 2. The effect of the short fibers here as well was an enhanced incorporation of matrix material in the respective plies. In a further treatment step fine residual porosity was simply and completely dealt with by further impregnation of S! polymer and its pyrolysis to give SiC. During practical testing as the tube of a canned motor pump it was 8 found that material had satisfactory sealing properties with respect to the liquid.

Example 4

For the production of a shaft guard sleeve for use in a pump for corrosive and abrasive media a cylindrical fiber member of SIC fiber strands was wound with diagonal crossing of the plies in relation to the axis of the cylinder up to a wall thickness of 1.5 mm followed by three cover plies in the circumferential direction. The supported fiber body was provided with a SIC matrix by means of CVL Thereafter a mixture of SIC powder with a mean particle size of 0.2 micron and an Si polymer were applied with a thickness of approximately 0.5 mm. By heat treatment a dense cover ply of SIC bound with SIC was formed, on which a surface of the desired quality was produced by grinding and polishing.

This composite ceramic component had optimum properties of its application owing to its combination of abrasion resistance and surface quality combined with the best possible fracture toughness and thus tolerance towards damage.

In a corresponding manner but with the reversed order of ply formation so that the inner surface of the cylindrical tube was sealed with the fiber-free SIC ply, a cylindrical lining for a mold filling device (filling bushing) of a synthetic resin injection molding machine was produced. The component proved to be satisfactory under complex conditions of operation such as a high filling pressure, an abrasive medium and changes in temperature.

j 1 9 9

Claims

1 A workplece of fiber reinforced ceramic material, including at least two juxtaposed fiber inserts which are manufactured of ceramic material and are surrounded by a matrix material, said fiber inserts having different structures and/or consisting of different materials.

2 A method for the production of a workpiece as claimed in claim 1, wherein fiber inserts of ceramic material are produced and juxtaposed to give a fiber composite member, which is impregnated with matrix material and is fired, said fiber inserts having a different structure and/or a different material.

3 A method as claimed in claim 2, wherein the composite frame structure is impregnated with the matrix material by liquid phase infiltration or by chemical gas phase infiltration.

4 A method as claimed in claim 2 or claim 3, wherein prior to the impregnation the composite fiber structure is caused to take up a liquid polymer material and is subjected to heat treatment.

A method as claimed in claim 4, wherein the liquid polymer material is applied in a pure liquid form with or without a solvent.

6 A method as claimed in claim 3, wherein the composite fiber structure is caused to absorb polymer material in an amount wh ich is just sufficient to span the fiber strands without shutting off pore cavities therei n.

7 A workpiece of fiber reinforced ceramic material, as claimed in claim 1 substantially as described above in the specification with reference to and as illustrated in the accompanying drawings.

8 A method of producing a workpiece of fiber reinforced ceramic material as claimed in claim 2 substantially as described above in the specification with reference to and as illustrated in the accompanying drawings.